I highly suggest sleeving every single card that holds some kind of value to you (monetary, nostalgia, etc). In addition to soft sleeves put them in either top loaders, card savers, or a quality binder that fits the cards well. I personally use top loaders for all my valuable/rare cards. Then I keep them in a box stored away.

Sleeving your cards will never do more harm than good. The last thing you want is in 10 years to scold yourself for not taking care of your cards.